This invention relates to a method for forming a wear-resistant layer on a surface of a metallic material such as, particularly, an inner surface of a cylinder or barrel, which is liable to wear, used for a plastic forming machine or a ceramic forming machine.
Conventionally, formation of a wear-resistant layer of this character has been carried out by methods including a method of surface nitriding treatment and a method of adopting a lining of wear resistant alloy formed by spray coating, building up, or centrifugal casting on the surface of a metallic material on which wear-resistant layer is formed. According to these conventional methods, however, it is considerably difficult to form the wear-resistant layer uniformly on the surface of an object which has a large dimension and on the inner surface of a hollow material having a small inner dimension, and, furthermore, a lined layer is liable to come off. Therefore, these methods are not satisfactory because of problems in the performance of the layer and the process of forming the same. There has also been proposed another method for eliminating the defects of the methods described hereinbefore, in which a wear-resistant layer is formed by bringing hard substance powde such as tungsten (WC) powder into contact with a base material or member directly or indirectly through an auxiliary layer, such as a partition plate made of an easily soluble material, interposed between the WC powder and the base member to form a filling-up layer, and by permeating melt of metal such as nickel (Ni)-based or cobalt (Co)-based self-melting alloy into the filling-up layer so as to disperse the fine grains of the hard substance such as WC into the metal. The thus formed wear-resistant layer is highly improved in the wear resisting property, and the metal can comparatively easily permeate through the filling-up layer if the filling-up layer of hard substance powder is formed by employing a suitable mold such as a core so that it is possible for this type of layer to be applied to the surfaces of base members having various outer configurations. If the thickness of this type of wear-resistant layer is increased, small vacancies or empty holes may be formed in the layer, which is considered to be caused by the contraction which occurs when the hard substance powder and the metal are sintered. When the thickness of the wear-resistant layer is 2 to 3 mm, vacancies having diameter of about 10 to 100 microns may be formed. In case the vacancies are extremely small in size, no substantial problem is caused, and the existence of such vacancies does not provide any problem or it is rather preferable since lublicating oil enters into the vacancies when, for example, they are formed in slide surfaces of machine tools. However, in the case of the inner surface of a cylinder of a plastic making machine, plastic enters into vacancies and stagnates in the same so that it is peeling off after being heated and burnt to be mixed into normal plastic.